Process for preparing polyamines for lighter color polyisocyanate based foam

ABSTRACT

The present invention encompasses an improved process for preparing polymethylene polyphenyl polyisocyanate comprising reacting phosgene with polymethylene polyphenyl polyamine, the improvement wherein the polymethylene polyphenyl polyamine is acidified prior to phosgenation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the process for preparing polyamineswhich are particularly useful in the preparation of polyisocyanates.More specifically, the present invention relates to the reaction ofaniline with formaldehyde in the presence of an acid to produce mixturesof polyamines which are particularly useful in preparing )lighter colorpolyisocyanates.

2. Brief Description of the Prior Art

Generally, it is known in the art to react aniline with formaldehyde inthe presence of an acid to produce polyamines, and to use saidpolyamines to produce polyisocyanates for use in the preparation offoams. Lately, the color of the foams has become one of the requirementsfor judging the quality of foams. Foams which are otherwise acceptablecan be rejected because of dark color.

Impurities in the polyamines which are used in preparing polyisocyanatesfor use in the preparation of foams have, at least in part, been blamedfor the discoloration in foams. Hence, research in this area has focusedon the preparation of polyamines which can be used in preparing lightercolor polyisocyanates and foams.

U.S. Pat. No. 4,792,624 discloses that polymethylene polyphenylpolyisocyanates of improved color can be obtained from certainpolyamines which are prepared by the following process. The processcomprises the preparation of the corresponding polymethylene polyphenylpolyamine by condensing aniline and formaldehyde in the presence of anacid catalyst which is characterized by adding a minor proportion of apolyamine mixture comprising di(aminophenyl)methanes and oligomericpolymethylene polyphenyl polyamines, (collectively known as polymericMDA to an intermediate stage of the condensation reaction where thevarious intermediately formed aminobenzylamines are present.

U.S. Pat. No. 4,465,639 discloses addition of controlled amounts ofwater to the reaction mixture produced by phosgenation of a mixture ofpolymethylene polyphenyl polyamines (and the like polyamines produced bycondensation of formaldehyde and aromatic amines) prior to completeremoval of excess phosgene gives rise to the corresponding polymethylenepolyphenyl polyisocyanates having significantly improved properties suchcolor of the polyisocyanates.

By the present invention, there is provided an alternate means ofobtaining lighter color polyisocyanates.

SUMMARY OF THE INVENTION

The present invention encompasses an improved process for preparingpolymethylene polyphenyl polyisocyanate comprising reacting phosgenewith polymethylene polyphenyl polyamine, the improvement wherein thepolymethylene polyphenyl polyamine is acidified prior to phosgenation.

In a presently preferred embodiment of the invention, the improvedprocess comprises preparing a polyamine mixture by reacting aniline withformaldehyde in the presence of an acid catalyst, followed byneutralizing the resultant polyamine mixture with a base, theimprovement comprising effectively acidifying the polyamine (more aptlydescribed as re-acidifying) to neutralize excess base and produce anacidic polyamine mixture prior to phosgenating the polyamine. In aparticularly preferred embodiment of the invention, the polyamine iscontinuously acidified by treating it with aqueous anhydrous hydrogenchloride, after neutralization and just prior to or during purification.

Advantageously, the amount of acid introduced by this process was suchas did not cause an equipment upset such as localized corrosion.

DETAILED DESCRIPTION OF THE INVENTION

The acidified polyamine mixture of the present invention is prepared byreacting aniline with formaldehyde in the presence of an acid catalystsuch as hydrochloric acid, followed by treating the polyamine mixturewith a base such as caustic soda and then re-acidifying the polyaminemixture.

Surprisingly, it has been found that in accordance with the presentinvention, the process step of acidifying the polyamine mixture undercertain conditions can result in a polyamine mixture which is useful inthe preparation of lighter color polyisocyanates and foams. It is adistinct feature of the invention that the step of acidifying thepolyamine occurs after the preparation and neutralization of thepolyamine with a base.

In the preparation of the polyamine mixture, aniline to formaldehyderatio can range from 1 to 50:1 and preferably 1.5 to 20:1. Illustrativebut non-limiting examples of the acid catalyst can be selected from thegroup consisting of hydrochloric, formic, sulfuric, acetic, andphosphoric acids and mixtures thereof. The acid catalyst is employed inan amount of 0.1 to 90 and preferably 0.2 to 50% protonation of theamine groups.

Aniline and hydrochloric acid are typically mixed together to form anamine hydrochloride solution. The degree of protonation (moles of acidto moles of aniline) can be from 0.1% to 90%, and is typically between0.2 to 50%. The aniline/aniline hydrochloride solution is thenefficiently mixed with the formaldehyde solution with the aniline toformaldehyde ratio varying between 1:1 to 50:1. These ratios areadjusted to alter the oligomer and isomer distribution of the finalproduct. The temperature of the reaction mixture is typically maintainedbelow 50° C. to eliminate side product formation. The reaction mixtureis then heated to a temperature in the range of 80°-120° C. to completethe reaction and conversion of aminobenzyl anilines and poly aminobenzylanilines (also known as intermediates) to di(aminophenyl) methanes andpolymethylene polyphenyl polyamines. After the reaction is complete, theacid is neutralized using a base (most typically sodium hydroxide). Alarge amount of salt is formed which is removed from the reactionmixture. Generally, the crude mixture of polyamines is washed with waterto remove residual salt. Finally, the material is purified to removeunreacted aniline and water.

In accordance with the present invention, the process step of acidifyingthe polyamine can be described as follows. In a preferred embodiment ofthe invention, the polyamine is acidified either batch-wise orcontinuously after purification by treating it with anhydrous hydrogenchloride or hydrochloric acid. In another embodiment of the invention,the improved process comprises preparing a polyamine mixture by reactinganiline with formaldehyde in the presence of an acid catalyst, followedby partial neutralization of the resultant polyamine mixture, theimprovement comprising effectively leaving the polyamine mixtureslightly acidic prior to phosgenating the polyamine.

After the acidification, the concentration of acid in the polyaminemixture is from 1 to 1000 and preferably 2 to 50 ppm acid. Whenanhydrous hydrogen chloride is used in acidifying the polyamine, thereoccurs significantly less corrosion in the reaction equipment.

Illustrative but non-limiting examples of the acids useful herein can beaqueous or anhydrous hydrogen chloride, sulfuric acid, acetic acid,phosphoric acid or a mixture thereof. Preferably, anhydrous hydrogenchloride is employed.

The condition under which the acidification is conducted can be asfollows:

    ______________________________________                                                                          DURING                                                                        PARTIAL                                             BEFORE       AFTER        NEUTRAL-                                    STAGE   PURIFICATION PURIFICATION IZATION                                     ______________________________________                                        TEMPER- 80-240° C.                                                                          80-120° C.                                                                          80-240° C.                           ATURE                                                                         TIME     0-1 hr.      0-14 days    0-4 hr.                                    AMOUNT   2-50 ppm     5-50 ppm     5-1000 ppm                                 ______________________________________                                    

In the preparation of the polyisocyanates, the reaction of phosgene withthe polyamine corresponding to the desired polyisocyanates, alternatelyreferred to as amine phosgenation, is conducted in the presence of aninert solvent such as chlorobenzene. The polyamines are reacted withphosgene in molar ratios of 1.5 to 20 moles and preferably about 2.2 to4.0 moles of phosgene per amine group. Upon completion of thephosgenation, the excess phosgene and hydrogen chloride formed areseparately or simultaneously removed. The phosgenation product which ispresent after the removal of excess phosgene and HCl is in the form of asolution and may be separated into a gaseous phase containing volatilecompounds having isocyanate groups and a liquid phase which issubstantially completely crude polyisocyanate. The liquid phase can beworked up to produce polyisocyanates in a purer state.

In the practice of the invention, the process can be employed inpreparing a variety of lighter color polyisocyanates. Thepolyisocyanates are denoted as lighter color polyisocyanates becausethey are useful in making lighter color foams. In the preparation of thefoams, the polyisocyanates are reacted with an isocyanate reactivecompound in the presence of a blowing agent.

The invention is further described by the following non-limitingexamples.

EXAMPLES Example 1

A sample of acidic polymethylene polyphenyl polyamine was prepared bypartially neutralizing the acid catalyzed reaction product of anilineand formaldehyde with less than the theoretical amount of sodiumhydroxide. After separation of the two layers, the polyamine waspurified distillation to remove unreacted aniline and water, resultingin material which contained an acidity of 60 ppm of HCl. This materialwas then phosgenated using chlorobenzene as a solvent to obtainpolymethylene polyphenyl polyisocyanate (PMDI) with a color ratingbetween 0.32 to 0.36 as measured by absorbance at 430 nm. The color of anormal sample of this polyisocyanate, produced by phosgenating polyaminewhich was completely neutralized is rated at 0.50. Thus, theacidification resulted in a 28-36% reduction in color.

Analysis of this material, compared to normal material is as follows:

    ______________________________________                                                  PMDI from partial neut.                                                                     Normal PMDI                                           ______________________________________                                        Color, 430 nm                                                                             0.349           0.494                                             % NCO       31.8            32.0                                              Acidity, ppm HCl                                                                          260             360                                               Viscosity, 25° C.                                                                  161             172                                               ______________________________________                                    

As shown below, applications testing of the polymethylene polyphenylpolyisocyanate produced from partially neutralized polymethylenepolyphenyl polyamine did not exhibit any unusual behavior when comparedto normal polyisocyanate in a number of polyurethane foam formulations.

    ______________________________________                                        System 1                                                                      Gel time, (sec.)  63      64                                                  Rise time, (sec.) 78      78                                                  Rise height, (cm.)                                                                              61.8    62.9                                                Density, (pcf.)   1.32    1.35                                                System 2                                                                      Gel time, (sec.)  130     131                                                 Rise time, (sec.) 144     152                                                 Rise height, (sec.)                                                                             66.7    67.6                                                Density, (pcf.)   1.36    1.36                                                ______________________________________                                    

Example 2

A sample of purified polymethylene polyphenyl polyamine was acidified to8 ppm with anhydrous hydrogen chloride. This material was allowed tostand for 3 days at about 100° C. before being continuously phosgenatedin chlorobenzene solvent to give the polymethylene potyphenylpolyisocyanate. For comparison, a sample of the same polymethylenepolyphenyl polyamine which was not treated with acid was phosgenatedunder the same conditions. Several samples of each material wereanalyzed and the results are given below as the average ± standarddeviation:

    ______________________________________                                                      PMDI in  PMDI in                                                              Acidic MDA                                                                             Control MDA                                            ______________________________________                                        Number of samples                                                                             7          5                                                  Color 430 nm    0.237 ± 0.007                                                                         0.288 ± 0.015                                   Color 520 nm    0.034 ± 0.001                                                                         0.047 ± 0.011                                   Acidity, ppm HCl                                                                              211 ± 26                                                                              170 ± 4                                         % NCO           32.50 ± 0.09                                                                          32.47 ± 0.05                                    Viscosity, 25° C.                                                                      133 ± 3 127 ± 5                                         Iron (ppm)      2.4 ± 0.1                                                                             3.5 ± 0.1                                       ______________________________________                                    

In essentially the same manner as described in Example 1, applicationand testing of the polymethylene polyphenyl polyisocyanate produced fromacidic polymethylene polyphenyl polyamine showed performance equivalentto that of the control sample.

Example 3

a) A system was designed to introduce aqueous hydrochloric acid into acrude polyamine stream (at about 100° C.) prior to purification. Theacid flow rate could be adjusted to deliver between 5 to 50 ppmhydrochloric acid in the final product after neutralizing the excessbase (see Table 1 below).

b) A system was designed whereby concentrated aqueous hydrochloric acidwas introduced into the purification section (at about 180° C.) andregulated with a rotameter. The acid flow rate could be adjusteddepending on production, and was also set to deliver between 5 to 50 ppmhydrochloric acid in the final product after neutralizing the excessbase (see Table 1).

c) Anhydrous hydrogen chloride was connected through a regulator to thepurification section (similar to section b) via stainless steel tubing.A needle valve was used to regulate the flow of anhydrous hydrogenchloride, and a flow tube was used to measure the flow. Flows were setto deliver between 5 and 50 ppm hydrochloric acid in the final productafter neutralizing excess base (see the Table below).

    ______________________________________                                        Phosgenation Results                                                          Run Number   430 nm    520 nm   Acidity                                                                             Method                                  ______________________________________                                        Non-Acidified                                                                 1            0.343     0.055    Basic                                         2            0.363     0.086    Basic                                         3            0.303     0.046    Basic                                         4            0.469     0.057    Basic                                         5            0.386     0.055    Basic                                         Avg. of individuals                                                                        0.384     0.057                                                  Acidified                                                                     1            0.298     0.042    10 ppm                                                                              i                                       2            0.300     0.055    12 ppm                                                                              ii                                      3            0.259     0.039    30 ppm                                                                              ii                                      4            0.239     0.051    20 ppm                                                                              iii                                     Avg of individuals                                                                         0.282     0.048                                                  Average Percent                                                                            -26%      -16%                                                   Change                                                                        Significance Level                                                                         >99.9%    >98.0%                                                 ______________________________________                                         i. Acidification of crude MDA with aqueous HCI (a)                            ii Acidification in the Purification section with aqueous HCl (b)             iii Acidification in the Purification section with anhydrous HCl (c)     

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. An improved process for preparing a polyamine byreacting aniline with formaldehyde in the presence of an acid catalyst,followed by neutralizing the resultant polyamine with a base, theimprovement comprising effectively acidifying the polyamine in an amountsufficient to produce an acidic polyamine after the neutralization. 2.The process of claim 1 wherein the step of acidifying the polyaminecomprises adding from about 2 to 5 ppm of anhydrous hydrochloric acid tothe reaction product of aniline and formaldehyde at a temperature ofabout 80° to 420° C.
 3. The process of claim 1 wherein the step ofacidifying the polyamine is conducted at a stage of purifying thepolyamine.
 4. The process of claim 3 wherein the stage of purifying thepolyamine comprises removing untreated aniline and water.
 5. The processof claim 1 wherein the acidification of the polyamine comprises adding asmall but effective amount of an acid to a base-neutralizedpolymethylene polyphenyl polyamine.
 6. The process of claim 1 whereinthe polyamine is acidified by partially neutralizing an acidifiedpolyamine.
 7. The process of claim 1 wherein the level of acidificationof the polyamine is from 1 to 1000 ppm.